The present application concerns photovoltaic devices, such as solar cell devices. More specifically, the present application concerns Group III-V compound semiconductor based photovoltaic devices employing a window layer.
A photovoltaic device converts light energy into electricity. Although the term “solar cell device” may sometimes be used to refer to a device that captures energy from sunlight, the terms “solar cell device” and “photovoltaic device” are interchangeably used in the present application regardless of the light source.
FIG. 1 is a cross-sectional view of a conventional multiple junction solar cell device 100. The multiple junction solar cell device 100 may include a substrate 101, a bottom cell unit 103, a middle cell unit 105, and a top cell unit 107. The multiple junction solar cell device 100 can be positioned to receive light from the front or top (illuminated) side of the device. The light typically include a plurality of wavelengths, and the cell units 103, 105 and 107 are typically designed to absorb different wavelengths of light. For example, the first range of wavelengths 111 may be absorbed in the bottom cell unit 103, the second range of wavelengths 113 may be absorbed in the middle cell unit 105, and the third range of wavelengths 115 may be absorbed in the top cell unit 107.
The multiple junction solar cell device 100 may also include a window layer 109 to improve the overall efficiency of the solar cell device 100. In the conventional multiple junction solar cell device 100, InAlP is widely used as a standard window layer. The window layer 109 is generally provided to prevent the surface recombination of photo-generated carriers. With the conventional window layer 109, the fourth wavelength 117, such as the far blue end or ultraviolet region of the solar spectrum, are absorbed in the window layer 109 so that the fourth range of wavelengths 117 are not transmitted to any of the cell units 103, 105 and 107 of the device 100. Therefore, the efficiency of the conventional device 100 decreases due to the window layer 109. Furthermore, one important mechanism for loss in the solar cell device is the recombination of photo-generated carriers, such as holes and electrons, at the top surface of the solar cell device due to the high density of surface states. The conventional window layer has a band gap of around 2.0 eV. A wider band gap would enhance the efficiency of the solar cell device 100 by reducing the recombination of the photo-generated carriers. However, it is difficult to grow materials with a band gap larger than 2.0 eV that are lattice matched to GaAs substrates in the conventional solar cell device 100.
Accordingly, a new solar cell structure is also needed with a window layer that minimizes surface recombination.